diff --git a/src/chunk.zig b/src/chunk.zig
index 56c7130..2db5177 100644
--- a/src/chunk.zig
+++ b/src/chunk.zig
@@ -61,6 +61,7 @@ pub const Chunk = struct {
         while (offset < self.count) {
             offset += self.dissassemble_instruction(offset);
         }
+        debug.print("== end of {s} ==\n\n", .{name});
     }
 
     pub fn dissassemble_instruction(self: Chunk, offset: usize) usize {
diff --git a/src/main.zig b/src/main.zig
index 0d3034a..a1b83b7 100644
--- a/src/main.zig
+++ b/src/main.zig
@@ -6,6 +6,8 @@ const Chunk = @import("./chunk.zig").Chunk;
 const OpCode = @import("./opcode.zig").OpCode;
 const VM = @import("./vm.zig").VM;
 
+pub const DEBUG_TRACE_EXECUTION = true;
+
 pub fn main() !void {
     var gpa = std.heap.GeneralPurposeAllocator(.{ .safety = true }){};
     defer _ = debug.assert(gpa.deinit() == .ok);
@@ -16,10 +18,10 @@ pub fn main() !void {
     var chunk = Chunk.new();
     try chunk.init(allocator);
 
-    try chunk.write(allocator, @intFromEnum(OpCode.OP_RETURN), 123);
     const constant = try chunk.add_constant(allocator, 1.2);
     try chunk.write(allocator, @intFromEnum(OpCode.OP_CONSTANT), 123);
     try chunk.write(allocator, @intCast(constant), 123);
+    try chunk.write(allocator, @intFromEnum(OpCode.OP_RETURN), 123);
 
     chunk.dissassemble("test chunk");
 
diff --git a/src/vm.zig b/src/vm.zig
index 78ca569..a2d8154 100644
--- a/src/vm.zig
+++ b/src/vm.zig
@@ -3,6 +3,8 @@ const Chunk = @import("./chunk.zig").Chunk;
 const OpCode = @import("./opcode.zig").OpCode;
 const Value = @import("./values.zig").Value;
 
+const DEBUG_TRACE_EXECUTION = @import("./main.zig").DEBUG_TRACE_EXECUTION;
+
 const print_value = @import("./values.zig").print_value;
 
 const InterpretResult = enum {
@@ -35,15 +37,25 @@ pub const VM = struct {
 
     pub fn run(self: *VM) InterpretResult {
         while (true) {
+            if (DEBUG_TRACE_EXECUTION) {
+                _ = self.chunk.?.dissassemble_instruction(self.ip.?);
+            }
+
             const instruction = self.read_byte();
 
             switch (instruction) {
                 @intFromEnum(OpCode.OP_CONSTANT) => {
                     const constant = self.read_constant();
+
+                    // XXX Those should not be std.debug, but stdout.
                     print_value(constant);
+                    std.debug.print("\n", .{});
                 },
                 @intFromEnum(OpCode.OP_RETURN) => return InterpretResult.OK,
-                else => return InterpretResult.RUNTIME_ERROR,
+                else => {
+                    std.debug.print("Invalid instruction: {d}\n", .{instruction});
+                    return InterpretResult.RUNTIME_ERROR;
+                },
             }
         }
 
@@ -53,9 +65,10 @@ pub const VM = struct {
     // XXX In the book, we're using a ptr to data directly, to avoid dereferencing to a given offset
     // How to do that in Zig?
     pub fn read_byte(self: *VM) u8 {
+        const ret = self.chunk.?.code[self.ip.?];
         self.ip.? += 1;
 
-        return self.chunk.?.code[self.ip.?];
+        return ret;
     }
 
     pub fn read_constant(self: *VM) Value {